A San Francisco-based cruise ship had more than 350 cases of norovirus in the past two months and is now being investigated by the federal government, but cruise owners say passengers are the ones bringing it aboard.

The Sea Princess reported 53 of its 2,196 passengers on the Princess Cruises 10-day Alaskan trip that left the Port of San Francisco on June 19 were affected by the virus, a highly contagious gastroenteritis infection. It is the fourth time since May 10 this ship has had an outbreak of the virus, according to the Centers for Disease Control.

There are no records of the Sea Princess reporting the virus prior to this year.

The first reported instance was aboard a 10-day cruise that left May 10. Forty-four of the 2,049 on board reported having the virus. On the May 20 cruise, 128 out of 2,053 passengers reported symptoms. Finally on the May 30 voyage, 142 out of 2,128 passengers were affected, according to the company.

According to the CDC, norovirus is generally spread through water or when touching surfaces contaminated by the virus. The virus is found in vomit or fecal matter of infected persons.

It is generally prevented through hand washing, proper food preparation and proper cleaning of infected surfaces. The CDC says most people with the virus suffer from diarrhea, stomach pain and vomiting, but recover within two days. More serious cases, however, can lead to death.

In 2010, six cruise ships from eight companies reported outbreaks of norovirus, according to the CDC. Five other ships in 2010 had diseases that were unknown.

In 2011, four ships from four companies have reported the virus, including the four instances on the Sea Princess and one on the Coral Princess, which is also operated by Princess Cruises. The Coral Princess also had reports of E. coli in April.

Princess Cruises spokeswoman Karen Candy said the company believes the June 19 passengers were exposed to the virus by a passenger who brought it aboard.

“In addition to our cleaning procedures, we rely on passengers’ compliance and good hygiene habits,” she said via email, “and the two of these must work in tandem to eradicate an outbreak.”

Candy said the Sea Princess went through a “prolonged and intensive cleaning” in recent days by the CDC and the cruise’s public health officers before it left for a 10-day Alaskan cruise Thursday.

Candy said the virus is not strictly a cruise ship virus; instead an estimated 23 million land-based cases are reported each year.

akoskey@sfexaminer.com

Sea sick

The Sea Princess has been afflicted with numerous norovirus cases in the past eight weeks.